The present invention relates to an exposure apparatus.
In order to meet the recent resolution improvement requirement, a projection exposure apparatus is proposed which projects a mask pattern onto a wafer using the extreme ultraviolet (“EUV”) light having a wavelength between 10 nm and 20 nm, and is referred to as a “EUV exposure apparatus” hereinafter.
A conventional illumination optical system applicable to the EUV exposure apparatus arranges an aperture stop having a semicircular aperture on a corrugated integrator for good illuminations with the small number of mirrors and no relay optical system. See, for example, Japanese Patent Laid-Open No. (“JP”) 2005-141158. The corrugated integrator is an integrator having plural cylindrical reflection surfaces having the same generatrix direction.
It is known that high-quality imaging with a projection optical system requires a restrained distortion of an effective light source of the illumination optical system viewed from each position on an illumination surface that is a target plane to be illuminated.
The illumination optical system disclosed in JP 2005-141158 sufficiently restrains the distortion of the effective light source for the prior art those days, but this inventor has studied and discovered that the illumination optical system in JP 2005-141158 still contains a remaining distortion that is not negligible.
The distorted effective light source of the illumination optical system in JP 2005-141158 results from partial shielding of the collimated light by the aperture stop before it forms the secondary light source. This phenomenon will be described in detail with reference to FIGS. 1 and 2.
FIG. 1 is a schematic perspective view of an arrangement of the aperture stop 15 and the integrator 11 in JP 2005-141158. An area 12 illuminated by collimated light CL on the integrator 11 narrows at an exit side of the aperture stop 15. FIG. 2 is a top view of the integrator 11 shown in FIG. 1. In order to obtain an effective light source having a regular circle at an end 21 in an arc illumination surface 20, the collimated light CL needs to be irradiated onto an area 13 shown by a dotted line.
At an incident side of the aperture stop 15, i.e., the lower half area in the top view of FIG. 1, the area 13 overlaps the area 12 and secures an area necessary for a regular circle of the effective light source. The aperture stop 15 shields the unnecessary light outside the area 13. The lower half area of the aperture stop 15 in FIG. 2 corresponds to the lower half area of the effective light source in FIG. 2, and the lower half area of the effective light source 22 has a semicircle with no distortion.
On the other hand, the exit side of the aperture stop 15, i.e., the upper half area in the top view of FIG. 2 contains a non-overlap area between the areas 13 and 12 and causes a shift from the regular circle of the effective light source. The area in the area 13 which receives no light causes an effective light source 22 to have a shape out of round. The unnecessary light outside the area 13 is not shielded because there is no subsequent stop, and the effective light source 22 has a shape that projects from the regular circle.
The configuration of the illumination optical system disclosed in JP 2005-141158 thus remains a slight distortion in the effective light source, and cannot meet a higher imaging characteristic requirement.